/*
 * Copyright 2013-2021 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.gateway.filter.headers;

import org.springframework.core.Ordered;
import org.springframework.http.HttpHeaders;
import org.springframework.web.server.ServerWebExchange;

/**
 * See https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.3 for details.
 *
 * 类的功能和背景
 * 		这个类实现了一个 HTTP 头部过滤器（HttpHeadersFilter），用于规范化 HTTP 请求头中的传输编码（Transfer-Encoding）和内容长度（Content-Length）。它主要用于处理在 HTTP 通信中可能出现的头部冲突问题。
 * 背景知识：
 * 		Transfer-Encoding:
 * 			这是一个 HTTP 头部字段，用于指定消息的传输方式。常见的值是 chunked，表示消息体以分块的方式传输。
 * 		Content-Length:
 * 			这个头部字段指定了消息体的字节长度。
 * 		规范要求:
 * 			根据 HTTP/1.1 协议，如果 Transfer-Encoding 的值为 chunked，那么 Content-Length 不应该同时存在。因为 chunked 模式下，消息体的长度是动态计算的，不需要 Content-Length。
 * 			因此，这个过滤器的作用是确保当 Transfer-Encoding 是 chunked 时，移除掉 Content-Length 头部，避免潜在的冲突或协议错误。
 */
public class TransferEncodingNormalizationHeadersFilter implements HttpHeadersFilter, Ordered {

	@Override
	public int getOrder() {
		return 1000;
	}

	@Override
	public HttpHeaders filter(HttpHeaders input, ServerWebExchange exchange) {
		String transferEncoding = input.getFirst(HttpHeaders.TRANSFER_ENCODING);
		if (transferEncoding != null && "chunked".equalsIgnoreCase(transferEncoding.trim())
				&& input.containsKey(HttpHeaders.CONTENT_LENGTH)) {

			HttpHeaders filtered = new HttpHeaders();
			// avoids read only if input is read only
			filtered.addAll(input);
			filtered.remove(HttpHeaders.CONTENT_LENGTH);
			return filtered;
		}

		return input;
	}

}
